【1049】Counting Ones (30 分)

#include<iostream>
#include<stdio.h>
#include<stdlib.h>
#include<math.h>
#include<string.h>
#include<algorithm>  
#include<map>
#include<vector>
#include<queue> 
using namespace std;  
//找到1~n的all数里含1的1的个数,对逐位分三种情况
//代码虽短，但要举例静心找规律
int main(){   
  int n,a=1,ans=0;
  int left,now,right;
  scanf("%d",&n);
  while(n/a != 0){
    left=n/(a*10);
    now=n/a%10;
    right=n%a;
    if(now == 0) ans +=left*a;
    else if(now==1)  ans += left*a+right+1;
    else ans += (left+1)*a;
    a*=10;
  }
  printf("%d\n",ans);
  system("pause");
    return 0;   
}